In the past two decades, tobacco expansion processes have become an important part of the cigarette manufacturing process. Tobacco expansion processes are used to restore tobacco bulk and volume which are lost during curing and storing tobacco leaf. Tobacco expansion processes are also used to increase the bulk of cured tobacco above that of the tobacco leaf in order to lower the "tar" and nicotine content of many cigarette products including low tar and ultra-low tar cigarettes.
Tobacco expansion processes involving contacting of tobacco with an impregnant followed by rapid heating to volatilize the impregnant and thereby expand the tobacco, are described in U.S. Pat. No. 3,524,451 to Fredrickson et al. and U.S. Pat. No. 3,524,452 to Moser et al. A process employing a vapor state impregnation of tobacco followed either by heating or rapid pressure reduction for tobacco expansion is disclosed by U.S. Pat. No. 3,683,937 to Fredrickson et al.
Carbon dioxide has been used in tobacco expansion processes as disclosed in U.S. Pat. No. 4,235,250 to Utsch; U.S. Pat. No. 4,258,729 to Burde et al.; and U.S. Pat. No. 4,336,814 to Sykes et al., among others. In these and related processes, tobacco is impregnated with carbon dioxide, either in gas or liquid form, and the impregnated tobacco is subjected to rapid heating conditions for expansion. These known carbon dioxide expansion processes, however, require excessive heating of the impregnated tobacco in order to achieve substantial and stable expansion. This excessive heating can harm the tobacco flavor and/or generate an excessive amount of tobacco fines. In addition, those processes which use liquid carbon dioxide for impregnating tobacco often result in impregnated tobacco in the form of solid tobacco blocks containing dry ice which must then be broken up prior to heat treatment. This can harm the tobacco and also increases the complexity and cost of the expansion process.
U.S. Pat. No. 4,531,529 to White and Conrad describes a process for increasing the filling capacity of tobacco, in which tobacco is impregnated with a low boiling, highly volatile expansion agent, such as a normally gaseous halocarbon or hydrocarbon, at process conditions above or near the critical pressure and temperature of the expansion agent. The pressure is quickly reduced resulting in expansion of the tobacco without the necessity of a heating step to either expand the tobacco or fix the tobacco in the expanded condition. The pressure conditions of this process range from 36 Kg/cm.sup.2 (512 psi) and higher with no known upper limit. Pressures below 142 Kg/cm.sup.2 (2000 psi) were used to produce satisfactory tobacco expansion without excessive fracturing. Normally gaseous hydrocarbons such as methane, ethane, and propane, are among the preferred impregnants used in this process.
U.S. Pat. No. 4,554,932 to Conrad and White describes a fluid pressure treating apparatus including a tubular shell housing a spool assembly. The spool includes a cylindrical body portion of relatively small diameter that extends between the two spool ends, which have a diameter greater than the spool body, but less than the diameter of the shell. The spool is mounted within the shell for reciprocating movement between a loading position outside the shell, a treating position within the shell, and an unloading position outside of the shell. When the spool is within the shell, deformable sealing rings carried in annular grooves on the cylindrical ends of the spool are forced radially outwardly for engagement with the interior of the shell. This provides a sealed, annular-shaped pressure chamber inside the shell, in the space between the spool ends surrounding the smaller spool body. One or more ports through the shell cooperate with conduit shaped cavities extending radially into the spool ends and axially along the spool body to allow input and removal of processing fluids into and from the annular space around the spool body within the shell. The use of this apparatus for high pressure impregnation of tobacco with an expansion agent permits rapid loading and unloading of tobacco and avoids the closure and opening problems associated with conventional pressure sealing and locking mechanisms, such as the pivoting autoclave lids of conventional pressure vessels. The spool and shell pressure vessel can thus produce time savings and improve economics in tobacco expansion.
Tobacco expansion processes, including those described above and others, must be conducted in batch processes when impregnation pressures substantially exceed atmospheric pressure. In order to achieve efficient and repeatable tobacco expansion in such batch processes, it is necessary to repeatedly form tobacco batches of a precise size based on the interior volume of the pressure treating vessel and/or density and expansion characteristics associated with the type of tobacco being expanded. Typically, design of the batch forming process is constrained by other manufacturing considerations associated with tobacco expansion processes including the desire to minimize leakage of volatile tobacco expansion agent as the tobacco is formed into batches and then fed and loaded into a pressure vessel for impregnation.